1. Field of the Invention
The present specification relates generally to a semiconductor laser driving apparatus for driving a semiconductor laser such as a laser diode and similar devices, and in particular to a semiconductor laser driving apparatus formed from a current mirror circuit capable of reading and writing information in an optical disc at a high speed while widely changing a control current.
2. Discussion of the Related Art
A conventional semiconductor laser driving apparatus generally supplies a laser diode LD with small to large currents with a current mirror circuit as shown in FIG. 11. Specifically, in the conventional laser driving apparatus, a pair of PMOS transistors Pa and Pb is connected to a power supply voltage VDD through their sources. Respective gates of the pair of PMOS transistors are connected at a connection point to each other. The connection point is connected to a drain of the PMOS transistor Pa. Thereby, a current mirror circuit is achieved.
A constant current source ISa is connected to the drain of the PMOS transistor Pa via a switch SWa, and thereby a constant current ia can flow thereto. Since the PMOS transistors Pa and Pb form a current mirror circuit, a current ib is supplied from the PMOS transistor Pb to the laser diode LD in accordance with a ratio between sizes of the PMOS transistors Pa and Pb. For example, when the ratio of the sizes of Pa and Pb is one versus two, a ratio between currents 1a and 1b is also one versus two.
Further, a control signal Sa input from a control circuit (not shown) turns ON/OFF the switch SWa. The current ib is controlled to be supplied in response to opening/closing of the switch SWa, and thereby the laser diode LD is turned ON and OFF. However, the laser diode LD does not irradiate a beam immediately after the switch SWa is turned ON and is closed.
Specifically, voltages of the gate and drain of the PMOS transistor Pa remains higher than a conduction level (i.e., VDD−Vth) initially, because the switch SWa is turned OFF and is open, and thus a current does not flow. The sign Vth represents a threshold voltage for the PMOS transistor Pa in the above.
Immediately when the switch SWa is turned ON in this situation, the current ia flows from the constant current source ISa, and thereby the gate and drain voltages start descending. However, since a parasitic capacitance Ca parasitizes with the PMOS transistors Pa and Pb, the switch SWa, and a wiring and so on, it takes a certain time to discharge some of charge stored in the parasitizing capacitance Ca, and a stand up performance of the output current ib saturates from when the switch SWa is turned ON to when the output current ib reaches a prescribed level as shown in FIG. 12. As a result, a delay time Tdr appears. Further, when the output current ib falls down to zero, a delay time Tdf appears.
Such saturation and delay of the stand up performance raises a problem, because it takes a certain time to discharge the parasitic capacitance Ca especially when the currents ib, and accordingly ia flowing from the constant current source ISa are small. Further, since each of source-gate voltages Vgs of the PMOS transistors Pa and Pb is small when each of the currents ia and ib is small, a difference ΔVth in a threshold voltage created by uneven manufacturing processes for manufacturing the PMOS transistors Pa and Pb apparently changes a ratio between currents ia and ib, which becomes a drawback.
Specifically, a current Idp flowing to a drain of a PMOS transistor is generally calculated by the following formula in a saturation range, wherein Kp is obtained by the following formula, and wherein μp represents a surface displacement degree of channel carrier, Cox represents a gate oxide film capacity, W represents a channel width, L represents a channel length, Vgs represents a souse-gate voltage (an absolute value), and Vth represents a threshold voltage (an absolute value).Idp=Kp×(Vgs−Vth)2/2  (a)Kp=μp×Cox×W/L 
When a small difference ΔVth takes place between the threshold voltages Vth of the PMOS transistors Pa and Pb due to uneven manufacturing processes, a difference in a current caused by the threshold voltage difference ΔVth is small, because the source-gate voltage Vgs is large when the current Idp is large. However, since the source-gate voltage Vgs is small, accordingly, the effect by the difference ΔVth is not negligible, and the above mentioned conduction level (Vgs−Vth) is duplicated when the current Idp is small, the smaller the source-gate voltage Vgs, the exponentially larger the difference in the current Idp.
Specifically, even if no problem occurs in the circuit of FIG. 11 when a large current flows into the laser diode LD, the slight difference ΔVth creates a considerable difference in the output current ib when a small current flows thereto, and thereby the current mirror circuit loses precision in current control, which becomes as a problem.